adjacent tiles within range

I made a script that highlights all tiles within a range. It works fine when it is far from the edges but it gives me " NullReferenceException: Object reference not set to the instance of an object" at line 83

Here is the script:

 public List<Tile> highlihtedTiles = new List<Tile>();
  public void HighlightAdjacentTiles(int xPos,int yPos,int _range)
     {
         int range = _range + 1;
         for (int x = -range; x < range; x++)
         {
             for (int y = -range; y < range; y++)
             {
                 if (xPos - x > 0 && yPos - y > 0 && xPos + x < mapSizeX && yPos + y < mapSizeY)
                 {
                     if ((Mathf.Abs(x - y) < range) && (Mathf.Abs(y + x) < range))
                     {
                         var result = tiles[xPos + x, yPos + y];
                             highlihtedTiles.Add(result);
                     }
                 }
             }
         }
         for (int x = 0; x < highlihtedTiles.Count; x++)
         {
             highlihtedTiles[x].GetComponent<SpriteRenderer>().color = Color.green;// line 83
             highlihtedTiles[x].highlightd = true;
         }
     }